Gregory Benford's Starborn

Bow shock "moisturization"

In Starborn, Benford suggests that if by some mischance the interface between the 50km/sec, 1/cm³ interstellar hydrogen reached in as far inward as the earth, the effects would be troublesome. Let's do the math.

1/cm³ is 1E6 / m³. So that is 5E10 hydrogens per square meter per second, or 8.3E-17 kg/m²-s (6.023E26 hydrogens/kg), or 2.62E-9kg/m²-year. That is coming from one direction, and the disk-to-surface ratio of a sphere is 1/4, so that averages 6.55E-10kg/m²-year.

Keep in mind that the earth swims in a sea of very hot and very thin hydrogen - that (slower) hydrogen is banging into the top of the atmosphere all the time, so the claim is questionable.

But if the interstellar hydrogen all entered the atmosphere, combined with oxygen, and turned to water, that would be 5.9E-9kg/m²-year. For comparison, the atmosphere averages about 25kg/m² of water in the tropospheric air column, and precipitation is about 1000 kg/m²-year.

Water freezes and (slowly) falls at the top of the troposphere, which defines the boundary with the stratosphere. The stratosphere is dry, not because water can't get there, but because it freezes and falls down into the troposphere.

Meanwhile, the air column weighs about 10,000 kg/m², and 2,000 kg/m² is oxygen. About 1% of it is in the stratosphere (and a tiny bit above in the mesosphere. So even if the stratosphere was not constantly getting replenished with oxygen from below, it would take 3.4 billion years before the stratosphere was emptied of oxygen by hydrogen appearing in a "galactic wind". In fact, it is replenished from the reservoir below, so it would take closer to 340 billion years.

But what about the ozone?

The stratosphere reaches from 10 to 50km altitude. Ozone forms in the lower stratosphere (it is destroyed by sunlight UV in the upper stratosphere) and is maximum at 20km altitude (about 5E18 molecule/m³). In my "US Standard Atmosphere 1976", I do not have hydrogen number densities for the stratosphere, but at 150 km the hydrogen density is 4E11/m³, the mean free path is about 100 meters, and the temperature is 270K, ( v(vert) = sqrt ( kT/m ) = 100 m/s for H2 ). So the hydrogen around earth produces 4E11 collisions per second at that very high altitude ( slower but denser than the interstellar wind ), about 30 times the the atomic flux rate of the interstellar wind. This does not destroy the ozone (down at 20km), where the hydrogen is even denser.

Comparison to Coronal Mass Ejections

Coronal mass ejections from the Sun have roughly similar density and speed, about 3 protons/cm³ and 500 km/sec . These can result in geomagnetic storms, causing field changes at the surface of the earth of 5 microteslas per second. The field changes can induce DC voltages and currents in power lines, causing transformers to saturate. However, these terrestrial problems are caused by a change in the particle flux. So the real problem would a rapid onset in particle flux (CMEs do this) and a similarly rapid magnetic change. It is hard to imagine a multi-AU sized event having the same rapid onset as a bubble of fast moving solar coronal plasma.

Conclusion

I have great respect for Dr. Benford as an author and as a plasma physicist, but I think the needs of the story won out over the physics. A collapse of the solar field sufficient to cause the earth to be directly impacted by interstellar gas would have very little effect. And since the field is a dipole, falling off by 1/R³, it is unlikely to move much.

OTOH, the particle flux necessary to collapse the solar field bow shock from 230 AU to 1 AU might imply interstellar gas densities jumping ten millionfold. The implied particle fluxes, ten million times higher than the analysis above, might produce serious effects, including significant extra drag on satellites up to high MEO. It is still doubtful that even that would affect the ozone layer noticably.